1. Field of the Invention
The present invention relates to a method of and an apparatus for generating halftone dots to reproduce a halftone image.
2. Description of the Related Art
In printing process, halftone images of four primary colors, that is, yellow (Y), magenta (m), cyan (C), and black (K), are printed with respective inks on a printing sheet to produce a color print. Each halftone image includes a large number of halftone dots which represent density variation in the image, and each halftone dot includes a number of painted pixels. The larger the number of painted pixels in a halftone dot, the larger the halftone dot; the larger the halftone dots, the darker the halftone image becomes.
The halftone image on the film is generally produced with a scanner which includes a reading unit for reading an original color image to capture color separation image signals, and a recording unit for exposing a photosensitive film to reproduce halftone images on the film. The color separation image signals represent density variations of the respective four primary colors in the original color image. The recording unit compares each image signal with specific screen pattern data, which represent threshold values, to generate an exposure control signal for each recording pixel as a result of the comparison, and exposes a halftone image on a photosensitive film while ON-OFF controlling a light beam in response to the exposure control signal.
FIG. 1 shows an example of screen pattern data for one halftone dot region, which is the maximum region of one halftone dot. In FIG. 1, one halftone dot region is made of 32.times.32 pixels, to each of which a threshold value ranging from 0 through 255 is assigned. The threshold value for each pixel is compared with the level of the image signal, and those pixels that have the threshold values no more than the level of the image signal are to be exposed. For example, when the level of the image signal is 64 over the 32.times.32 pixels, the shaded areas at the corners of the halftone dot region are exposed to make halftone dots. If the level of the image signal is 255, all pixels in the halftone dot region are exposed.
When superimposed to make a color print, the four halftone images might cause a so-called "moire". orientation angles, or screen angles, of the four halftone images are generally set at different values to prevent the moire. FIGS. 2A through 2D show halftone dot arrangements where the screen angles .theta. are set at 0 degrees, 15 degrees, 45 degrees, and 75 degrees, respectively.
The production of the four dot arrangements with different screen angles .theta. can be executed by two typical methods, that is, Rational Tangent Method and Irrational Tangent Method. Tangent of each screen angle .theta. (tan .theta.) is set at a rational number in the Rational Tangent Method, and it is set at an irrational number in the Irrational Tangent Method.
FIG. 3 is a conceptive view illustrating an arrangement of screen pattern data for the screen angle .theta. of about fifteen degrees according to the Rational Tangent method. FIG. 3 includes four halftone dot regions. U denotes a primary scanning direction and V denotes a secondary scanning direction. In the Rational Tangent Method, screen pattern data is prepared such that it is addressed along each primary scanning line. Even if a scanning line SL runs through three halftone dot regions HD1, HD2, and HD3, the screen pattern data is successively read out along the scanning line SL. The Rational Tangent Method therefore requires four sets of screen pattern data for the four screen angles, respectively, so that the screen pattern data can be addressed along the primary scanning line at each screen angle.
It is sometimes required to change a screen ruling, or the number of halftone dots per inch, of the halftone image. The screen ruling is changed by adjusting a diameter and an interval of beam spots of a light beam to be focused on the photosensitive film in the Rational Tangent Method. This, however, requires a conglicated shuttle mechanism moving in the secondary scanning direction and an expensive optical system which can change the diameter and interval of the beam spots.
The Irrational Tangent Method, on the other hand, can change the screen ruling relatively easily. The Irrational Tangent Method needs only one set of screen pattern data which are assigned to pixels in one halftone dot region as shown in FIG. 1, and reads out the screen pattern data as a function of the screen angle .theta.. FIG. 4 illustrates a method of reading out screen pattern data in the Irrational Tangent Method. X-Y coordinates indicate an address of a screen pattern memory, or a memory for storing screen pattern data, and U and V denote primary and secondary scanning directions, respectively. The U-V coordinates of an arbitrary point A are transformed into the X-Y coordinates as follows: EQU X=U*cos .theta.+V*sin .theta. (1a) EQU Y=-U*sin .theta.+V*cos .theta. (1b)
where "*" denotes multiplication. When assuming U=m*p and V=n*p, Equations (1a) and (1b) are rewritten as follows: EQU X=m*p*cos .theta.+n*p*sin .theta. (2a) EQU Y=-m*p*sin .theta.+n*p*cos .theta. (2b)
where "m" and "n" are integers and "p" denotes a pitch of recording pixels, or a side length of a recording pixel.
Since the secondary scanning coordinate v is constant on each primary scanning line, the integer "n" is also constant on each scanning line. Therefore, only the first terms of Equations (2a) and (2b) change in reading out the screen pattern data along each scanning line. According to Equations (2a) and (2b), one-pixel progress in the primary scanning direction increases the X coordinate by p*cos .theta. and the Y coordinate by p*sin .theta.. Accordingly, the screen pattern data for only one halftone dot region shown in FIG. 1 is sufficient to produce halftone dot arrangements at any screen angle in the Irrational Tangent Method. Furthermore, the screen ruling is also changeable by adjusting the pixel pitch p.
FIG. 5 shows the relationship between the arrangement of recording pixels and the address on the screen pattern memory in the Irrational Tangent Method. Recording pixels RP shown as circles are disposed along scanning lines SL1 and SL2. Addresses of the screen pattern data are positioned at the centers of the grid shown in FIG. 5. The coordinate axes of the X-Y coordinate systems run through the center of the address (0, 0) accordingly.
The scanning coordinates (U, V) for each recording pixel are transformed into the address coordinates (X, Y) according to the above Equations (2a) and (2b) . In the example of FIG. 5, screen pattern data stored at the address (0, 0) , (2, 1) , and (3, 1) are successively read out when a series of recording pixels RP disposed on the first scanning line SL1 are to be exposed. In a similar manner, when a series of recording pixels RP disposed on the second scanning line SL2 are to be exposed, screen pattern data stored at the address (0, 2) and (1, 2) are read out. In the Irrational Tangent Method, the address on the screen pattern memory does not go along the primary scanning direction U, and some addresses on the screen pattern memory are skipped in producing halftone dots. If tan .theta. is an irrational number, positional relation between a recording pixel and a corresponding address of the screen pattern memory varies at each recording pixel. Therefore the addresses which are referred to in a certain halftone dot region are generally different from those referred to in adjacent halftone dot regions.
Since the positional relation between a recording pixel and a corresponding address of the screen pattern memory varies with respect to each halftone dot region, the size of halftone dots, or the number of pixels in the halftone dots, fluctuates accordingly even if those halftone dots are produced as a function of an image signal of a constant level. FIG. 6 shows an example of plural halftone dots generated as a function of an image signal with a constant level. Lattice points in FIG. 6 correspond to the centers of recording pixels RP, and circular regions Rex represents the address range of the screen pattern data which are no more than the level of the image signal, that is, the address range in which the recording pixels are exposed. Incidentally, the coordinate axes of the U-V systems run through the center of the recording pixel at an origin O.
The screen pattern data is read out by the address corresponding to the center of the recording pixel RP, which is shown as a lattice point in FIG. 6. The screen pattern data thus read out is compared with the image signal to determine whether the recording pixel RP is exposed or not. As a result, the recording pixels whose centers are located in the region Rex are exposed, and the other pixels are not exposed. The exposed pixels are shown as shaded polygons in FIG. 6. The number of the exposed pixels in a halftone dot ranges from 9 to 12 in this example.
In the Irrational Tangent Method, variation in the positional relation between the address of the screen pattern data and the recording pixels causes fluctuations in size and shape of halftone dots. In other words, halftone dots which are produced as a function of an image signal of a constant level do not have the same area. The variation in the positional relation increases the darkness in some image areas while increasing the lightness in other image areas, thus causing uneven and unstable image reproduction.
Several methods have been proposed to suppress the unevenness of a reproduced image. A method which adds a random number to the address bas been proposed, but this method has a disadvantage of deforming halftone dots. Another method is to shift or distort each halftone dot region at random. The second method, however, could not efficiently suppress fluctuations in size and shape of halftone dots. There is still another method which changes the distribution of light quantity of a light beam. The third method, however, requires a relatively complicated optical system for changing the light quantity of the light beam.